This invention relates to optical time-domain reflectometry (OTDR) and is more particularly concerned with an arrangement in which optical time-domain reflectometry is used to interrogate different portions of an optical fibre extending through a region of interest. The variable to be sensed (the measurand) may be temperature, but other variables could be sensed using similar techniques. Optical fibre sensors can offer the considerable advantages of immunity from electrical interference and the danger of sparks such as might be produced in electrical sensors. Distributed optical fibre sensors are of particular interest since they allow the value of the measurand to be determined at many separate points along the entire length of an optical fibre, thus enabling many independent measurements to be made with a single apparatus.
Distributed sensing in optical fibres is discussed in UK Patent Application No. GB 2 122 337 A and also in other technical literature (e.g. in Proceedings of the 8th European Conference on Optical Communication, Cannes, 1982, A. H. Hartog and D. N. Payne, and in Journal of Lightwave Technology, vol LT-1, pp 498-509, 1983, A. H. Hartog). In these disclosures, the principle of sensing with optical fibres using techniques of optical time-domain reflectometry that had already been established for the detection of faults in optical fibre communications links was demonstrated. Essentially, it was shown that light scattered during the propagation of a pulse of light launched into a fibre at one end, and returned to the launching end by guidance within the fibre, can contain information concerning the distribution along the fibre of a variable of interest (e.g. temperature).
GB 2 122 337 A is concerned mainly with a liquid-filled fibre in which the scattering loss coefficient varies with temperature, thus causing the intensity of the returning scattered radiation to vary in accordance with the local temperature. The time elapsing between the launching of the light pulse and the detection of a sample of the scattered light gives the location of the measurement point. It has been found, however, that similar effects can be observed in solids, and can be obtained not only from modulation of the scattering loss but also, for example, of the numerical aperture of the fibre. Modulation of the total loss is considered in the prior disclosures, but is not of importance to the present application.
Another prior specification (GB 2 140 554 A) dealing with OTDR proposes that the central part of the spectrum of the scattered radiation should be filtered out by means of a so-called optical dichrometer to leave only two spectral regions of interest, namely the regions occupied by the so-called Raman Stokes lines and Raman Anti-Stokes lines. The local temperature is said to be obtainable by comparing the respective optical intensities of back-scattered light in these two spectral regions. However, to obtain best results from this method it would be necessary to correct the measurements for the difference between the fibre attenuations at the various Stokes and Anti-Stokes wavelengths.